The invention relates to a process for the preparation of copolymers of carbon monoxide with one or more compounds comprising an ethylenically unsaturated bond.
It is known that copolymers in which the units originating from carbon monoxide substantially alternate with the units originating from the ethylenically unsaturated compound(s), may be prepared by contacting the monomers under polymerization conditions in the presence of a suitable catalyst.
Convenient methods for preparing such copolymers are disclosed in prior art references, such as EP 181014 and EP 248483. The copolymers obtained according to these, or similar preparation methods, are relatively high molecular weight compounds, having established utility as thermoplastics in the production of shaped articles, such as containers for foods and drinks and parts for the car industry and for various applications in the domestic sphere.
For these uses and the like, the copolymers have to be melt processed, i.e. they have to be brought in molten state so that they can be molded or extruded. In view of their high melting point, usually of 200.degree. C. or above, the melt stability of the copolymers may become a problem if the copolymers are to remain in molten state for a substantial length of time. The longer the copolymers are subjected to high temperatures, the more they are susceptible to degradation caused by chemical reactions.
The formation of degradation products will usually be at the expense of the properties of the copolymer product.
The presence of specific impurities in the copolymers may have a negative effect on the stability of the copolymers while being processed at high temperatures.
The origin of these impurities is not always known, but in many cases they are apparently introduced in the reactor, together with the starting materials, the catalyst or with the diluent or solvent, if any.
It has already been proposed to reduce the content of typical impurities, such as sulfur in sulfidic form and iron present as iron carbonyl, which impurities often occur in the carbon monoxide feedstock, in particular if the carbon monoxide used is a commercial grade feedstock, such as commonly available at (petro)chemical sites.
While a reduction in the content of these impurities in the carbon monoxide monomer feed has a favorable effect on the average reaction rate at which the polymerization proceeds, it has little effect on the melt stability of the copolymers obtained.
It has now been found that the melt stability of the copolymers of the invention is adversely affected by the presence therein of alkali metal and/or alkaline earth metal impurities, hereinafter sometimes referred to as metal impurities, and that a significant improvement in melt stability is achieved by reducing the metal impurities content of the produced copolymers.
It has also been found that a significant improvement in melt stability is achieved by reducing the alkali(ne earth) metal salt content of the produced copolymers or by adding to a copolymer which is contaminated with alkali(ne earth) metal salt a substance which is able to bind the contaminating salt. Evidently, both treatments may be combined as well.
The detrimental influence of minute quantities of alkali(ne earth) metal salts on the melt stability of the copolymers is surprising in view of the prior art. For example, in EP-A-285218 it is disclosed to treat the copolymers with alkali metal N,N-diethyldithiocarbamate as a step in reducing the concentration of palladium or nickel catalyst remnants for the purpose of improving the copolymers' melt stability. In U.S. Pat. No. 3,948,850 it is disclosed to stabilize ethylene/carbon monoxide copolymers having up to 50% mol carbon monoxide by adding thereto a dihydro phosphoric acid salt of an alkali(ne earth) metal.
Calcium hydroxyapatite and zeolites containing a metal of Group 2 of the Periodic Table, such as calcium, have been recommended as metal stabilizers for the linear alternating copolymers of carbon monoxide and ethylenically unsaturated compounds. The present invention surprisingly teaches that trace quantities of the alkali(ne earth) metal salts which may be present in the polymers need to be substantially reduced or removed or need to be bound in order to stabilize the copolymers, as opposed to the prior art which teaches that these metals have to be added.